1. Field of the Invention
The present invention relates to an apparatus for detecting an electric potential distribution on a photoconductor caused by a laser beam exposure, and also to an image density control apparatus for executing AIDC by using the result of the detection of the electric potential distribution.
2. Description of the Prior Art
The present inventor has previously disclosed in Japanese patent laid-open number 4-271779 an imaging method that compensates for changes in gradation characteristics when the beam diameter of the laser beam varies due to changing the laser diode. The previous method changes the maximum strength of the laser beam according to the beam diameter after laser diode replacement, or changes the .gamma. tables (tables of gradation correction data) according to the beam diameter after laser diode replacement.
In light intensity modulation methods achieving a gradation display by varying the laser light quantity in plural steps, an exposure distribution as shown in FIG. 1 occurs in subscanning direction y. The main scanning direction x and exposure i are also shown in FIG. 1.
The exposure distribution .rho. (i,x,y) is obtained as EQU .rho.(i,x,y)=i.rho..sub.a (y) (1)
where the normalized light quantity distribution in the subscanning direction is r.sub.a and the average light quantity is i. Sample results obtained with this equation are graphed in FIG. 2.
The potential distribution (attenuation potential) V(i,y) of the electrostatic latent image formed on the photoconductor by exposure at the above light distribution i.rho..sub.a (y) is obtained by equation (2) below. ##EQU1## where V0 is the charge potential, VR is the residual potential, and k is the sensitivity constant. Sample results obtained with this equation are graphed in FIG. 3.
In a light intensity modulation gradation display, there is a specific exposure distribution in the subscanning direction y as described above, resulting in display problems as illustrated in FIGS. 4-7.
FIG. 4 shows the correlation between the potential distribution of each latent image when the laser beam diameter is large (left side of figure) and small (right side), and the amount of toner adhering to the photoconductor by development of the latent image at developer potential VB when the average exposure i level is low (low image density). Note that the toner quantity is proportional to the difference between the developer potential VB and the attenuation potential (the potential of the latent image).
From FIG. 4 it is understood that toner adheres more easily to the photoconductor at a low exposure i because the potential valley is deeper with a small diameter laser beam. In other words, toner adhesion to the photoconductor begins at a lower average exposure i.
FIG. 5 shows the correlation between the potential distribution of each latent image when the laser beam diameter is large (left side of figure) and small (right side), and the amount of toner adhering to the photoconductor by development of the latent image at developer potential VB when the average exposure i level is high (high image density).
From FIG. 5 it is understood that in the high average exposure i region toner easily adheres uniformly when a large diameter laser beam is used because potential valleys do not occur easily, but that overall toner adhesion decreases when the laser beam diameter is small because potential valleys occur easily.
The characteristics shown in FIGS. 4 and 5 are graphed in FIG. 6. FIG. 6 plots the relationship between toner density (adhesion) and exposure (light quantity) when the beam diameter W1/2 is varied in seven levels at 5 .mu.m increments from 45-75 .mu.m. The axis of abscissas in FIG. 6 shows the 256 gradation light levels assuming a maximum light level set to a known value, and the axis of ordinates shows the toner density (toner adhesion).
From FIG. 6 it is understood that toner adhesion can be varied by adjusting the beam diameter W1/2 while maintaining a constant (same) light quantity (same average exposure i) under certain conditions. In other words, toner density increases when the light quantity is low and the beam diameter W1/2 is small because toner adhesion occurs even at relatively low exposure levels with a small beam diameter as shown in FIG. 4. When the exposure level is high and the beam diameter W1/2 is large, toner density also increases because toner adheres to the overall image when the beam diameter W1/2 is large as shown in FIG. 5. It is to be noted that toner density is essentially unchanged irrespective of the beam diameter W1/2 at intermediate gradation levels around an exposure (light quantity) level of 72.
The relationship between toner density and light quantity adjusted in 256 gradations is shown in FIG. 7 assuming the maximum toner adhesion is controlled to a constant value by AIDC as described below. These curves are shown for the same beam diameters W1/2 used in FIG. 6.
From FIG. 7 it is understood that the difference in toner density caused by differences in beam diameter W1/2 is corrected by AIDC at high exposure (average exposure i) levels, but this difference in toner density is more pronounced at low exposure levels.
Thus, particularly with the light intensity modulation method, technologies correcting the variation in gradation characteristics as disclosed in Japanese patent laid-open number 4-271779 are available because the gradation characteristics are affected by the beam diameter W1/2. As indicated by the properties discussed with reference to FIGS. 4 and 5 above, however, the gradation characteristics are not varied simply by the change in beam diameter W1/2, but are more precisely varied by the change in the potential distribution of the latent image caused by differences in beam diameter W1/2.
Therefore, the method described in Japanese patent laid-open number 4-271779 cannot cope with differences in the potential distribution of the latent image occurring when the beam diameter W1/2 remains constant. FIG. 21b shows the potential distribution (dotted line in FIG. 21b) of the ideal latent image that should be formed when the photoconductor is exposed with a laser beam with a light quantity distribution as shown in FIG. 21a, and the potential distribution (solid line in FIG. 21b) of the latent image that is actually formed. There is an obvious difference between the two curves. This difference in potential distribution is caused by a difference in the light exposure diffusion due to surface condition of the photoconductor or by a difference in the charge diffusion due to a difference of the surface treatment. These differences occur individually from variations during photoconductor manufacture, environmental conditions, and deterioration of the photoconductor with use. It is not possible to treat sufficiently the problem by using only the gradation correction due to the a difference in the beam diameter.
FIGS. 19a and 19b show an electric potential distribution formed by an equal beam diameter. As mentioned above, in a case when there is a large amount of diffusion due to the difference in the surface condition of the photoconductor, the distribution shows a small potential difference (.DELTA.Vs) between the summit and the valley as shown in FIG. 19a. On the other hand, the distribution shows a large potential difference (.DELTA.Vs) when there is a small amount of diffusion as shown in FIG. 19b. Accordingly, there occurs a gradation variation due to an electric potential difference (.DELTA.Vs) between the summit and the valley in the potential distribution as shown in FIG. 20 in a similar way to that of gradation variation due to the beam difference as shown before. That is, in a case when the maximum density is made constant, a large amount of electric potential difference (.DELTA.Vs) permits the low density part rises up at an earlier time and a small amount of electric potential difference (.DELTA.Vs) permits the low density part rises up at an later time.
Therefore, in view of the problem mentioned above, the present invention is to achieve a good gradation display by compensating for changes in the gradation display due to differences in the electric potential difference caused by a difference in the beam diameter, light diffusion, or charge diffusion.
To achieve this object, an image density control apparatus according to the present invention comprises a means for forming a latent image having a potential distribution by scanning and exposing the surface of a uniformly charged photoconductor with a laser beam; a means for detecting the potential distribution of the latent image; a means for storing the potential distribution; a means for calculating a potential difference gap (potential difference between the max. and min.)
An image density control apparatus according to the present invention forms a standard latent image by means of a laser beam emitting a known light quantity to the surface of a photoconductor charged to a uniform potential by a charger controlled to a known grid potential; develops a toner image of this latent image by means of a developer controlled to a known developer potential; detects the density of this standard toner image; optimizes the image density during the final imaging process by controlling the grid potential and developer potential according to the density of this standard toner image; and comprises a means for detecting the electric potential distribution; a memory for storing the gradation correction data in accordance with each of plural potential distribution and in accordance with the each of densities of the toner image as a reference; a first selection means for selecting the gradation correction data group corresponding to the electric potential distribution (potential difference gap) detected by the potential distribution detection means; and a second selection means for selecting the table corresponding to the standard toner image density from the group of tables selected by the first selection means.
The electric potential distribution detector stores the electric potential distribution of the charge latent image formed on the photoconductor with the laser beam and then calculates the potential difference (.DELTA.Vs) between the max value (summit value) and the min. value (valley value) in accordance with the stored potential distribution.
The image density control apparatus manages the first selection means to select the gradation correction data corresponding to the potential difference (.DELTA.Vs) detected by the electric potential distribution detector. Next, the second selection means selects the gradation correction data corresponding to the detected density from the data group selected by the above first selecting means.
The image density is controlled based on the data obtained from these selected gradation correction data.